Xerox Network Systems

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The Xerox Network Systems (XNS) protocols were created by the Xerox Corporation in the late 1970s and early 1980s. They were designed to be used across a variety of communication media, processors, and office applications. Several XNS protocols resemble the Internet Protocol (IP) and Transmission Control Protocol (TCP) entities developed by the Defense Advanced Research Projects Agency (DARPA) for the U.S. Department of Defense (DoD).

Background

Because of its availability and early entry into the market, XNS was adopted by most of the early LAN companies, including Novell, Inc.; Ungermann-Bass, Inc. (now a part of Tandem Computers); and 3Com Corporation. Each of these companies has since made various changes to the XNS protocols. Novell added the Service Advertisement Protocol (SAP) to permit resource advertisement and modified the OSI Layer 3 protocols (which Novell renamed IPX, for Internetwork Packet Exchange) to run on IEEE 802.3 rather than Ethernet networks. Ungermann-Bass modified RIP to support delay as well as hop count, and made other small changes. Over time, the XNS implementations for PC networking have become more popular than XNS as it was designed by Xerox. This article presents a summary of the XNS protocol stack in the context of the OSI reference model.

Figure: Xerox Adopted a Five-Layer Model of Packet Communication

As illustrated in the above figure, Xerox provided a five-level model of packet communications. Level 0 corresponds roughly to OSI Layers 1 and 2, handling link access and bit-stream manipulation. Level 1 corresponds roughly to the portion of OSI Layer 3 that pertains to network traffic. Level 2 corresponds to the portion of OSI Layer 3 that pertains to internetwork routing, and to OSI Layer 4, which handles interprocess communication. Levels 3 and 4 correspond roughly to the upper two layers of the OSI model, handling data structuring, process-to-process interaction, and applications. XNS has no protocol corresponding to OSI Layer 5 (the session layer).

Media Access

Although XNS documentation mentions X.25, Ethernet, and High-Level Data Link Control (HDLC), XNS does not expressly define what it refers to as a Level 0 protocol. As with many other protocol suites, XNS leaves media access an open issue, implicitly allowing any such protocol to host the transport of XNS packets over a physical medium.

The following descriptions summarize the IDP packet fields illustrated in the following figure:

Checksum - A 16-bit field that helps gauge the integrity of the packet after it traverses the internetwork.

Length - A 16-bit field that carries the complete length (including the checksum) of the current datagram.

Transport control - An 8-bit field that contains the hop count and Maximum Packet Lifetime (MPL) subfields. The Hop Count subfield is initialized to 0 by the source and is incremented by 1 as the datagram passes through a router. When the Hop Count field reaches 16, the datagram is discarded on the assumption that a routing loop is occurring. The MPL subfield provides the maximum amount of time, in seconds, that a packet can remain on the internetwork.

Figure: Eleven Fields Comprise an IDP Packet

Packet type - An 8-bit field that specifies the format of the data field.

Destination network number - A 32-bit field that uniquely identifies the destination network in an internetwork.

Destination host number - A 48-bit field that uniquely identifies the destination host.

Destination socket number - A 16-bit field that uniquely identifies a socket (process) within the destination host.

Source network number - A 32-bit field that uniquely identifies the source network in an internetwork.

Source host number - A 48-bit field that uniquely identifies the source host.

Source socket number - A 16-bit field that uniquely identifies a socket (process) within the source host.

IEEE 802 addresses are equivalent to host numbers, so hosts that are connected to more than one IEEE 802 network have the same address on each segment. This makes network numbers redundant but nevertheless useful for routing. Certain socket numbers are well known, which means that the service performed by the software using them is statically defined. All other socket numbers are reusable.

XNS supports Ethernet Version 2.0 encapsulation for Ethernet, and three types of encapsulation for Token Ring: 3Com, SubNet Access Protocol (SNAP), and Ungermann-Bass.

XNS supports unicast (point-to-point), multicast, and broadcast packets. Multicast and broadcast addresses are further divided into directed and global types. Directed multicasts deliver packets to members of the multicast group on the network specified in the destination multicast network address. Directed broadcasts deliver packets to all members of a specified network. Global multicasts deliver packets to all members of the group within the entire internetwork, whereas global broadcasts deliver packets to all internetwork addresses. One bit in the host number indicates a single versus a multicast address. Conversely, all ones in the host field indicate a broadcast address.

To route packets in an internetwork, XNS uses the RIP dynamic routing scheme. Today, RIP is the most commonly used Interior Gateway Protocol (IGP) in the Internet community.

Transport Layer

OSI transport layer functions are implemented by several protocols. Each of the following protocols is described in the XNS specification as a Level 2 protocol.

The Sequenced Packet Protocol (SPP) provides reliable, connection-based, flow-controlled packet transmission on behalf of client processes. It is similar in function to the Internet Protocol suite's Transmission Control Protocol (TCP) and the OSI protocol suite's Transport Protocol 4 (TP4).

Each SPP packet includes a sequence number, which is used to order packets and to determine whether any have been duplicated or missed. SPP packets also contain two 16-bit connection identifiers. One connection identifier is specified by each end of the connection; together, the two connection identifiers uniquely identify a logical connection between client processes.

SPP packets cannot be longer than 576 bytes. Client processes can negotiate use of a different packet size during connection establishment, but SPP does not define the nature of this negotiation.

The Packet Exchange Protocol (PEP) is a request-response protocol designed to have greater reliability than simple datagram service (as provided by IDP, for example) but less reliability than SPP. PEP is functionally similar to the Internet Protocol suite's User Datagram Protocol (UDP). PEP is single packet-based, providing retransmissions but no duplicate packet detection. As such, it is useful in applications where request-response transactions can be repeated without damaging data, or where reliable transfer is executed at another layer.

The Error Protocol (EP) can be used by any client process to notify another client process that a network error has occurred. This protocol is used, for example, in situations in which an SPP implementation has identified a duplicate packet.

Upper-Layer Protocols

XNS offers several upper-layer protocols. The Printing Protocol provides print services, the Filing Protocol provides file access services, and the Clearinghouse Protocol provides name services. Each of these three protocols runs on top of the Courier Protocol, which provides conventions for data structuring and process interaction.

XNS also defines Level 4 protocols, which are application protocols. However, because they have little to do with actual communication functions, the XNS specification does not include any pertinent definitions.

The Level 2 Echo Protocol is used to test the reachability of XNS network nodes and to support functions such as that provided by the ping command found in UNIX and other environments.

Summary

XNS is used as a protocol today only in networks by vendors that adopted some of the standards that XNS provides. These number are shrinking still, and very few new networks are being based upon XNS.

Review Questions

Q - When and where were the XNS protocols created?

A - The Xerox Network Systems (XNS) protocols were created by Xerox Corporation in the late 1970s and early 1980s.